An acute or chronic infectious disease of humans and some animals,
tuberculosis (TB) is caused by the bacterial organism
Mycobacterium
tuberculosis, often called the tubercle bacillus. One of the oldest known infections of humans, it has also been known as consumption or the great white plague. Human
infection with M. tuberculosis was one of the leading causes of death until the 1940s, when antituberculosis antibiotics were developed. It is still a major public health problem on a worldwide basis.
TB spreads when a person infected with the active form of the disease coughs droplets carrying the TB organism into the air, which are then inhaled by other people. The disease is not spread through a single exposure to infected droplets but from extended contact with an infected person. Once inhaled, the TB organism usually remains in the lung. The disease may also be acquired by drinking unpasteurized animal milk. In the vast majority of cases the infection is localized and symptomless. Most individuals who are infected with the TB organism do not develop active disease because their immune systems encapsulate and sequester the organism in the lungs, forming what is known as a granuloma. In this state, the organism can remain latent for years; however it may become active and progress to chronic pulmonary tuberculosis if left untreated. The test for TB infection involves injecting the skin with purified protein derivative (PPD) of the TB organism. Swelling at the site of injection indicates the presence of infection.
Typical symptoms of active TB infection include fatigue, loss of weight and appetite, night sweats and fever, and persistent cough. Sputum is often streaked with blood, and massive hemorrhage can occur. Untreated, the disease leads to gradual deterioration and weight loss and sometimes death. TB may sometimes spread from the lungs via the bloodstream to any organ in the body such as the brain, lymph nodes, bones, joints, kidneys, skin, and genital organs. Miliary TB, a rapidly progressive multiorgan form of the infection, is rare in the United States. Why some individuals develop active TB while others do not is not entirely understood. Underlying chronic diseases such as malnutrition, AIDS, cancer, and diabetes make development of active disease more likely.
From the time of the development of antibiotics to treat TB up to the mid-1980s, there was a significant and consistent drop in the annual number of cases reported in the United States. Since then there has been a resurgence in TB in the United States, first noticed among prison populations and in AIDS patients. This increase has been attributed in large part to the growth in the number of people with AIDS, which weakens the immune system. Other causes include an increase in the number of immigrants to the United States from parts of the world where TB is common, as well as increased crowding in urban areas.
Tuberculosis is treated with a combination of antibiotic drugsÑusually three or fourÑsuch as isoniazid, rifampin, pyrazinamide, and ethambutol, which are taken for 6 to 24 months. However, new drug-resistant strains of TB have developed, in large part because some patients do not follow the long-term treatment regimen once they feel better, allowing the bacteria to develop resistance. Public health officials have begun a program for patients who are not as likely to complete treatment, in which these patients take their medications in front of health-care workers, to make sure the medication is taken. Treatment with prophylactic courses of antibiotics may be appropriate for individuals who have had a positive PPD skin test but have not developed active disease. A vaccine known as BCG (Bacillus Calmette-Guerin) prepared from a living but weakened strain of bacterium may confer some protection against TB and has been used extensively in the developing world.
Sequencing of the bacterium's genome, or complete DNA molecule, was accomplished in 1998. Thisnotable breaking of the genetic code of the disease offers the hope that more fully successful anti-TB drugs and vaccines may be developed in the future.